1. Field of the Invention
The invention in general relates to sonar systems, and more particularly to a side-looking sonar system with all range focusing and increased search rates.
2. Description of the Prior Art
In side-looking sonar systems, a transducer on a carrier vehicle periodically projects pulses of acoustic energy toward a target area such as the sea bottom. The transmitted energy has a beam pattern which is of extremely narrow width in the direction of carrier travel so that a projected pulse of acoustic energy initially impinges upon the sea bottom and sweeps out a relatively narrow elongated area known as the insonified area.
Acoustic energy reflected back from the insonified area, and targets on it, is received by a receiver transducer. The receiver beam associated with the receiver transducer is similar to the transmitted beam such that the receiver detects reflected acoustic energy from the relatively narrow insonified area. The received acoustic energy is processed and displayed on a suitable display apparatus. With each pulse transmission and subsequent reception a scan line is produced on the display to build up a picture of the sea bottom in a manner similar to the scanning of a conventional cathode ray beam in a television picture tube.
With the insonified area being swept out to the side, or sides of the carrier vehicle, the resolution in the direction of vehicle travel is limited by the length of the receiver transducer. For increased resolutions there has been developed a side-looking sonar transducer which is of a curved configuration wherein the transducer segments making up the transducer lie along the arc of a circle whose radius is the design altitude and acoustic energy is focused along a line of focus on the sea bottom. These focused transducers however must be used at a precise altitude above the sea bottom since excursions above or below this design altitude tend to defocus the operation and consequently degrade the display. Proposals to obviate this design limitation have included the use of electronic focusing which involves a plurality of fixed delays in series with the transducer segments and wherein these delays are switched into the circuit after predetermined time durations so that focusing occurs in steps. Although this method is a step toward freeing the transducer from a critical altitude, the switching in of the delays results in a display which objectionably changes its gray scale periodically.
The maximum Search rate S of a side-look sonar which achieves a resolution r (generally the width of the insonified strip) is given by S = Nrc. Where N is the number of beams per side, and c is the speed of sound in the water. If the search rate is to be improved for a given resolution, several beams must be formed in the time it takes for a single pulse to travel out to the maximum range and return. The multi-beam capability also allows for the maintaining of a certain search rate with improved resolution. Multiple beam side looking sonar systems have been developed such as illustrated in U.S. Pat. No. 3,742,436, however such systems are constrained to a particular design altitude above the target area, plus or minus some depth of field.